1. Field of the Invention
This invention relates to an indexable cutter insert for a rotary cutter such as a face milling cutter.
2. Prior Art
One conventional cutter insert 10 of the indexable type shown in FIGS. 1 to 3 comprises a plate of a generally square shape defined by a front face 12, a rear face 14 disposed parallel to the front face 12, and four side faces 16. The four corners of the insert 10 are removed to provide four corner faces 18. The insert 10 has four main cutting edges 20 each defined by the front face 12 and a respective one of the side faces 16, and four auxiliary cutting edges 22 each defined by the front face 12 and a respective one of the corner faces 18. That corner portion where each adjacent side face 16 and corner face 18 intersect is chamfered to provide a chamfered surface 24, and each chamfered surface 24 cooperates with the front face 12 to define a corner cutting edge 26.
FIGS. 4 to 6 show a face milling cutter 28 employing a plurality of inserts 10 of the type described. The milling cutter 28 includes a generally disc-shaped body 28a having a plurality of pockets or recesses 30 formed in a forward end face 32 thereof in circumferentially spaced relation to one another. Each insert 10 is received in a respective one of the pockets 30 and fixed thereto by a clamping wedge 34 and a retaining plate 36 between which the insert 10 is sandwiched. In this condition, the indexed auxiliary cutting edges 22 are disposed slightly beyond the forward end face 32 of the cutter body 28a and substantially in a common plane perpendicular to the axis X of rotation of the cutter body 28a. The indexed main cutting edge 20 of each insert 10 disposed adjacent to the indexed auxiliary cutting edge 22 and directed radially outwardly of the cutter body 28a serves as a peripheral cutting edge of the milling cutter 28. As is well known in the art, the auxiliary cutting edge 22 of the inserts 10 serve to provide a finish surface of a workpiece processed by the milling cutter 28. Therefore, it is necessary that the indexed auxiliary cutting edges 22 of the inserts 10 should be located in their respective indexing positions, that is to say, in the common plane perpendicular to the axis X of rotation of the cutter body 28a as accurately as possible so as to provide a satisfactory finish surface of the workpiece. Each corner face 18 of the conventional insert 10 shown in FIGS. 1 to 4 is ground so that the auxiliary cutting edge 22 adjacent to the corner face 18 is disposed as close to its indexing position as possible when the insert 10 is attached to the cutter body 28a. Since each corner face 18 is formed by a substantially flat surface, the entire corner face 18 must be ground despite the fact that the corner face 18 actually has only to be ground at that portion adjacent to the auxiliary cutting edge 22. This is undesirable from an economical point of view because much labor and time are required for the grinding of the corner face 18.
In order to overcome the above difficulty, there has been proposed another conventional insert 10a shown in FIGS. 7 and 8 which differs from the above-mentioned insert 10 in that a rearward portion of a peripheral surface remote from a front face 12 is offset inwardly throughout the entire periphery thereof to provide a clearance portion 40. The peripheral surface 40a of the clearance portion 40 serves as a clearance surface. With this conventional insert, it is necessary to grind only a forward portion of each corner face 18 remote from a rear face 14 to provide a required auxiliary cutting edge 22. This saves time and labor. However, since the clearance portion 40 is provided around the entire periphery of the insert 10a, the strengths of the main, auxiliary and corner cutting edges 20, 22 and 26 are lowered. This problem is serious particularly with respect to the main cutting edges 20 because they are subjected to an increased cutting load during the cutting operation and therefore are susceptible to breakage or damage. In addition, the peripheral surface 40a of the clearance portion 40 is disposed perpendicular to the front and rear faces 12 and 14 so that a clearance depth D of the clearance portion 40 is relatively large. This not only aggravates the problem of breakage of the main cutting edges 20 but also affects the auxiliary and corner cutting edges 22 and 26.